In the conventional manufacture of paper sheets for use in tissue, towelling and sanitary products, it is customary to perform, prior to drying, one or more overall pressing operations on the entire surface of the paper web as laid down on the Fourdrinier wire or other forming surface. Conventionally, these overall pressing operations involve subjecting a moist paper web supported on a papermaking felt to pressure developed by opposing mechanical members, for example, rolls. Pressing generally accomplishes the triple function of mechanical water expulsion, web surface smoothing and tensile strength development. In most prior art processes, the pressure is applied continuously and uniformly across the entire surface of the felt. Accompanying the increase in tensile strength in such prior art papermaking processes, however, is an increase in stiffness and overall density.
Furthermore, the softness of such conventionally formed, pressed and dried paper webs is reduced not only because their stiffness is increased as a result of increased interfiber hydrogen bonding, but also because their compressibility is decreased as a result of their increased density. Creping has long been employed to produce an action in the paper web which disrupts and breaks many of the interfiber bonds already formed in the web. Chemical treatment of the papermaking fibers to reduce their interfiber bonding capacity has also been employed in prior art papermaking techniques.
A significant advance in producing lower density paper sheets, however, is disclosed in U.S. Pat. No. 3,301,746 which issued to Sanford et al. on Jan. 31, 1967, said patent being hereby incorporated herein by reference. The aforesaid patent discloses a method of making bulky paper sheets by thermally predrying a web to a predetermined fiber consistency while supported on a drying/imprinting fabric and impressing the fabric knuckle pattern in the web prior to final drying. The web is preferably subjected to creping on the dryer drum to produce a paper sheet having a desirable combination of softness, bulk, and absorbency characteristics.
Other papermaking processes which avoid compaction of the entire surface of the web, at least until the web has been thermally predried, are disclosed in U.S. Pat. No. 3,812,000 which issued to Salvucci, Jr. et al. on May 21, 1974; U.S. Pat. No. 3,821,068 which issued to Shaw on June 28, 1974; and U.S. Pat. No. 3,629,056 which issued to Forrest on Dec. 21, 1971, the aforesaid patents being hereby incorporated herein by reference.
All of the aforementioned patents disclose lowdensity papermaking processes and products wherein the web is not stratified. Applicants, however, have unexpectedly discovered that layering of papermaking fibers to form a stratified web can be employed to particular advantage when utilized in combination with such low-density papermaking processes. This is accomplished by subjecting the web to a fluid force while supported on an intermediate drying/imprinting fabric at relatively low fiber consistencies to produce soft, bulky and absorbent paper sheets of unusually high caliper and unusually low density, said paper sheets being particularly suitable for use in tissue, towelling and similar products.